1. Technical Field
The present invention relates to a system for RFID communication.
2. Background Information
Antennas are known from “Rothammels Antennnbuch”, A. Krischke, 12th edition, 2001. Different forms of antenna are explained on pages 65 to 71. The purpose of the antenna is to convert the line wave coming from the transmitter into the airborne wave or conversely to pick up the airborne wave from the air and to convert it into the line wave which is then passed to the receiver. The antenna is a transmission area which may be envisaged as a spread line. It acts as a matching transformer between line and air. With power adaptation, which is sought in the case of transmission and reception, a travelling wave is formed.
On pages 107 to 111 two-wire cables are explained, which consist of two wires extending parallel, with a small spacing relative to the wavelength. Two-wire cables, or double cables also known as parallel-wire cables, are formed symmetrically to ground. Striplines and microstriplines are explained on page 112. If a loss-free line is terminated at its end with a load resistor which corresponds to the line wave resistor, then the power running into the terminating resistor is completely consumed. This situation is the ideal adjustment. The adjustment factor is the reciprocal value of the ripple. The adjustment factor assumes the value 1 in the event of adjustment and the value 0 in no-load conditions or short-circuit. According to pages 118, 119, power supply lines which transmit high frequency tend to act as antennas. The radiation emitted in their surroundings can cause undesirable directionality and losses. Radiating power supply lines can also give rise to disruptions of the radio and television reception. This side-effect is usually more unpleasant than the low radiation loss. The unwanted radiation from power supply lines is dependent on the one hand upon the structure of the line and on the other hand upon the degree of mismatching on the line, and it increases with increasing ripple. Two-wire cables are symmetrical to ground, both individual conductors have the same cross-section and the same ground conditions. Therefore the currents flowing in both conductors are of equal magnitude, but in opposite directions. The magnetic fields behave in an analogous manner. They would cancel each other out if both conductors were to coincide spatially, but this cannot be achieved in practice. Because of the physical spacing always present between the two conductors the cancellation is not complete. The radiation loss from a two-wire cable increases directly with the square of the conductor spacing and the operating frequency. This means that the conductor spacing should be smaller as the frequency increases.
Balancing elements are explained on pages 145 to 155. If there is a phase rotation of 180°, the synchronized waves cancel each other out and the push-pull waves intensify. A distinction is made between tuned and broadband balancing elements, and between non-transforming and transforming balancing elements. Balancing and transformation are often carried out simultaneously.
In U.S. Pat. No. 7,298,267 B2, a system for RFID communication and for testing is known. In this case, a RF source is designed to provide RF energy to a RFID transponder. The RF source emits a continuous RF signal via a transmission line. Separately from this a coupler for coupling to the RFID transponder and a diode are provided, wherein the diode is coupled to the coupler and an interface. The interface is coupled to the diode and adapted by means of the coupler to modulate the RF energy of the RF source. By the separation of the RF source from the interface, which modulates the RF energy by means of the coupler, a simple arrangement with a plurality of couplers is formed which enables parallel testing of a large number of RFID transponders at high speed.